1. Technical Field
The invention relates to a method of operating an electron beam measuring device.
2. Related Art
An electron beam measuring device is known from Appl. Phys. Lett. 51 (2), 1987, pages 145 to 147 in which the thermal LaB6 or field emission source was replaced by a photocathode acted upon by a pulsed laser beam (pulse repetition frequency .gamma.=100 MHz, pulse width .apprxeq.1 to 2 ps). Since the width of the photoelectron pulses which are generated corresponds approximately to the width of the laser pulses, this device is particularly suitable for stroboscopic measurements in fast gallium arsenide circuits. The expenditure on apparatus for generating the photoelectron pulses is, however, considerable. The frequency of the laser light (.lambda.=1064 nm) has to be doubled twice in order to release photoelectrons from the gold layer which acts as the cathode (work function of electrons W=4.5 eV).
A scanning electron microscope is known from J. Phys. E: Sci. Instrum 20 (1987), pages 1491 to 1493 in which the tungsten cathode is heated with the aid of a pulsed laser beam within a few nanoseconds to temperatures above the melting point and is thereby stimulated to thermal emission of electrons. In this device too, doubling of the frequency of the primary radiation generated in a Nd-YAG laser is necessary. In addition, at higher laser energy levels a second electron pulse occurs which has to be screened out with the aid of a deflecting system. Also, because of the high evaporation rate of the cathode material the operational life of the cathode is limited.